This invention relates generally to communications systems, and, in particular to a receiver that performs adaptive channel equalization.
CDMA systems are based on a digital, wideband, spread spectrum technology which transmits multiple, independent user signals across an allocated segment of the radio spectrum. In CDMA, each user signal includes a different orthogonal code and a pseudo random binary sequence that modulates a carrier, spreading the spectrum of the waveform, and thus allowing a large number of user signals to share the same frequency spectrum. The user signals are separated in the receiver with a correlator which allows only the signal with the selected orthogonal code to be despread. The other user signals, whose codes do not match, are not despread, and as such, contribute to system noise. The signal to noise ratio of the system is determined by the ratio of the desired signal power to the sum of all interfering signals, enhanced by the system processing gain and the ratio of the spread bandwidth to the baseband data rate.
In the downlink (base station to mobile terminal) of a cellular direct-sequence code division multiple access (DS-CDMA) system, various users are multiplexed to the channel typically by orthogonal spreading codes. This is also the case with the 3rd generation wideband CDMA (WCDMA) standards currently proposed, where different spreading factors and variable user data rates can be supported simultaneously. However, due to multipath propagation and frequency-selective fading, the orthogonality between the various users"" waveforms is degraded, and multiple access interference impairs the performance of the receiver. For an uplink (mobile terminal to base station) receiver, several multiuser detection schemes have been proposed in order to alleviate multiple access interference. However, the mobile terminal cannot accommodate the same degree of computational complexity as the base station.
Several adaptive algorithms, based on stochastic gradient methods and minimum mean-squared-error (MMSE) criterion, have been proposed as a means of low-complexity interference suppression for CDMA receivers. In the presence of known training data sequences, a least-mean-square (LMS) algorithm can be used.
Reference in this regard may be had to S. L. Miller, xe2x80x9cAn Adaptive Direct-Sequence Code-Division Multiple Access Receiver for Multiuser Interference Rejection,xe2x80x9d IEEE Transactions on Communications, vol. 43, pp. 1746-1755, February-April 1995.
Adequate training for the LMS can also be provided by a conventional RAKE receiver, as is disclosed in M. Latva-aho, Advanced receivers for wideband CDMA systems. Doctoral thesis, Department of Electrical Engineering, University of Oulu, Finland, 1998.
In order to avoid the need for training, blind adaptation schemes have been proposed as described in: M. L. Honig, U. Madhow and S. Verdxc3xa, xe2x80x9cBlind adaptive multiuser detection,xe2x80x9d IEEE Transactions on Information Theory, vol. 41, pp. 944-960, July 1995, and also described in: N. Zecevic and J. H. Reed, xe2x80x9cBlind adaptation algorithms for direct-sequence spread-spectrum CDMA single-user detection,xe2x80x9d IEEE International Vehicular Technology Conference, VTC""97, May 1997, pp. 2133-2137.
Y. Bar-Ness and J. B. Punt, xe2x80x9cAdaptive bootstrap CDMA multi-user detector,xe2x80x9d Wireless Personal Communications, Kluwer Academic Publishers, vol. 3, no. 1, pp. 55-71, 1996, and P. Komulainen, Y. Bar-Ness and J. Lilleberg, xe2x80x9cSimplified bootstrap adaptive decorrelator for CDMA downlink,xe2x80x9d IEEE International Conference on Communications, ICC""98, Atlanta, USA, June 1998, pp. 380-384 both disclose algorithms based on blind signal separation, and these algorithms are shown to have equal performance with a linear MMSE receiver. It should be noted however, that in multipath channels, the blind adaptation schemes need some form of channel response estimation. For channel estimation, either a common pilot channel or dedicated pilot symbols can be utilized.
The formerly proposed adaptive approaches focus on the detection of the data symbols, thus requiring the signal to be cyclostationary at the symbol level. Unfortunately, this excludes their application to systems employing long pseudonoise (PN) scrambling codes, which change the correlation properties of the signal from one symbol interval to another.
In most digital radio communication systems, intersymbol interference (ISI) is introduced due to multipath propagation in the channel. This problem is significant especially at high data rates, and it may be alleviated by channel equalization.
Reference in this regard may be had to D. P. Taylor, G. M. Vitetta, B. D. Hart and A. Mxc3xa4mmelxc3xa4, xe2x80x9cWireless channel equalisation,xe2x80x9d European Transactions on Telecommunications, vol. 9, no. 2, pp. 117-143, 1998.
In the CDMA downlink, multiple access interference is essentially caused by the channel, since all the users"" signals propagate through the same frequency-selective multipath channel to the receiver of interest. Therefore, the orthogonality between the users can be restored and interference may be suppressed by compensating for the channel effects as disclosed in: S. Werner and J. Lilleberg, xe2x80x9cDownlink channel decorrelation in CDMA systems with long codes,xe2x80x9d IEEE International Vehicular Technology Conference, VTC""99, Houston, Tex., May 1999.
It is an object and advantage of this invention to provide an improved receiver which utilizes an adaptive channel equalization function to restore the orthogonality between different users"" waveforms.
It is a further object and advantage of this invention to utilize an adaptive channel equalization function in the receiver which operates to perform linear interchip interference chip cancellation by adaptive chip separation, by decorrelating consecutive transmitted chips.
It is a further object and advantage of this invention to provide a receiver with improved performance which is suited for systems using long scrambling codes such as third generation WCDMA systems.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A receiver for use in a CDMA telecommunications system is disclosed. The receiver includes at least one antenna for receiving signals from a CDMA channel, where the received signals include a desired user signal. The receiver also includes combining circuitry, for performing chip waveform filtering and maximal ratio combining, to produce mutually correlated chip estimates of the received signals. The receiver further includes an adaptive separator, for adaptively separating the mutually correlated chip estimates, and a correlator, for despreading the output of the adaptive separator to obtain an estimate for data symbols of the desired user signal.
In addition, the receiver further includes estimating circuitry, coupled to the combining circuitry, for estimating a response of the channel, where the combining circuitry utilizes the channel response estimate as a reference.